Yeast host expression systems have been used to express and secrete proteins foreign to yeast. Numerous approaches have been developed in terms of the degree of expression and the yield of biologically active mature proteins.
Such approaches have involved modifications to the various molecular components that are involved in expression and secretion of proteins in yeast. These components include the translation and termination regulatory regions for gene expression; signal peptide and secretion leader peptide sequences, which direct the precursor form of the heterologous protein through the yeast secretory pathway; and processing sites, which cleave leader peptide sequences from the polypeptide sequence of the protein of interest.
Expression of the protein of interest can be enhanced with use of yeast-recognized regulatory regions. Increased yield of the heterologous protein of interest is commonly achieved with the use of yeast-derived signal and secretion leader peptide sequences. The use of native signal-leader peptide sequences is believed to improve direction of the protein of interest through the secretory pathway of the yeast host.
Previous work has demonstrated that full-length yeast .alpha.-factor signal-leader sequences can be used to drive expression and processing of heterologous proteins in yeast host cells. Substantial improvements in efficiency of expression can be accomplished with the use of truncated .alpha.-factor leader sequences, particularly for heterologous proteins that are poorly expressed by the full-length sequence or whose expression is nonresponsive to the full-length sequence.
Although the various approaches available in the art have been shown to work with some proteins, problems persist with post-translational processing. Often the amount of protein secreted is unacceptably low or incorrect processing leads to inactive forms of the protein. This is particularly true for proteins that are initially expressed as a precursor polypeptide sequence and whose assumption of a native conformation is facilitated by the presence of a native propeptide sequence in the precursor polypeptide.
Methods for expression of heterologous proteins and their secretion in a biologically active mature form using a yeast host cell as the expression system are needed.